The present invention relates to a friction clutch and method, and more particularly, to a pressure-medium actuated clutch with exclusively axially moving friction discs engaged by at least one engagement spring and disengaged by a pressure medium disengagement actuator of the axial piston type overcoming the engagement spring.
A friction clutch is disclosed in DE-OS 38 31 005 as located in the power transmission path between a central output shaft and a hollow countershaft, concentric with the latter, of a gearwheel change-speed gearbox. The countershaft can be driven in the gearbox by a parallel main shaft via a gearwheel stage for a medium forward gear and via a gearwheel stage for a reverse gear by a gear-change clutch interacting with the loose wheels of these gearwheel stages. The main shaft includes the fixed wheels of the gear wheel stages and is drive by a driven engine via the usual main clutch.
A pressure medium actuated plate clutch with stationary annular cylinder whose piston acts on a pressure plate via a plate spring against the force of a disengagement spring is described in DE-AS 25 40 191. This plate clutch is of a different type than a pressure-medium actuated clutch of the type disclosed in DE-OS 38 31 005 because a disengagement spring is used for disengagement, and the piston does not overcome the engagement spring during the disengagement procedure. In this known plate clutch, the annular cylinder is a double-acting cylinder. In the region between the piston and the pressure plate, a self-locking device in the form of a ball catch is provided on a hub connected to one clutch half for clutch locking (only releasable by a pressure pulse) in the engaged condition. The self-locking device consists of several interlocks arranged on a pressure ring rotationally connected to the piston. Each of these interlocks is provided with an opening in which a ball, located in a hole in the hub and subjected to the force of a spring, engages. This special construction is intended to permit, in such a known plate clutch, a bearing configuration which is intended to make high rotational speeds possible with long life. Due to the self-locking device, it is also intended that actuation should only be possible by pressure pulses via two separate pipes. In this case, it is regarded as an advantage that only one single double-acting cylinder is necessary for closing and opening the clutch.
In vehicles with manual gearboxes, the main clutch is used for separating the gearbox from the engine during the gear-changing procedure, which can only take place in the load-free condition, and as a slipping element when driving away. These main clutches are usually single-plate dry clutches which are closed by compression springs. The main clutch is opened by a stationary actuating element and the opening force acts on the rotating main clutch via a thrust bearing.
In vehicles with automatic gearboxes, friction clutches are used for changing gear under load. The friction clutches are usually wet plate clutches which are hydraulically actuated. The actuating piston rotates with the friction clutch, which is open in the unpressurized condition and is closed by the supply of pressurized oil to the actuating cylinder. The advantage of the manual gearbox main clutch is that it is closed mechanically for the main driving range. It is only opened for brief periods to change gear. The energy requirement for actuating the clutch is small. In contrast, pressure must be supplied continuously in the case of automatic gearboxes in order to keep the engaged friction clutch sufficiently well closed for the gearbox input torque to be transmitted. This requires a large amount of energy.
In existing automatic gearboxes, the required pump power amounts to about a third of the total gearbox losses. If friction clutches in automatic gearboxes were constructed in such a way that they were closed without pressure, i.e. if the torque could be transmitted by spring force, the gearbox losses could be reduced by some 20 to 25%. This would increase the gearbox efficiency by approximately 2%. The difficulty with this is that the engagement spring must be configured for the maximum torque to be transmitted which, on one hand, causes large scatter in the spring force and, on the other hand, demands a high release for every gear change, this again being subject to scatter. The effect of this is to reduce the quality of the changes under load. This is particularly significant at small engine torques where, under certain circumstances, the scatter can be much greater than the needed control magnitude.
The losses in gear-changing quality and the complex construction of the automatic gearbox as an epicyclic gearbox in which, depending on the gearbox configuration, several friction clutches and/or friction brakes are always opened and others closed, have previously led to friction clutches in automatic gearboxes being constructed in such a way that they are open in the unpressurized condition. In twin clutch gearboxes (i.e., two-path gearwheel change-speed gearboxes of the countershaft type) which do not require any brakes for gear changing under load but only require two friction clutches which transfer the power alternately, both friction clutches can also be constructed in an arrangement in which they are closed when unpressurized. The engine torque is transmitted to the driven shaft via the power-transmitting gearbox branch by the closed friction clutch and the synchronization of the gear selected. The free gearbox branch also rotates at engine speed with the second friction clutch closed but is separated from the driven shaft by the open synchronization systems.
An object of the present invention is to provide a clutch arrangement, preferably for a twin-clutch gearbox, which is mechanically closed for the main driving ranges, i.e. requires little pump power, and which simultaneously ensures a gear-changing quality which is comparable with that of present-day epicyclic gearboxes.
In a friction clutch in which friction discs are engaged by at least one engagement spring and disengageable via a pressure medium disengagement actuator of the axial piston type, the foregoing object has been achieved advantageously in accordance with the present invention by providing a pressure medium actuator of the axial piston type, and the engagement spring of the friction discs can also be disconnected by the disengagement actuator when the friction discs are actuated in the engagement direction by the engagement actuator.
In the present invention, a friction clutch mechanically to reduce the power loss is closed by an engagement spring. The friction clutch is disengaged by a disengagement actuator which can rotate with the friction clutch and is supplied with pressurized oil via a hydraulic rotating joint or which is attached to a stationary casing part. The disengagement force is transmitted to the engagement spring via a thrust bearing.
The necessary gear-changing quality is achieved by an engagement actuator which acts on the friction clutch and closes the clutch when subjected to pressure. The engagement actuator can also rotate with the friction clutch or be attached to a fixed location and the engagement force can be transmitted to the friction clutch via a thrust bearing. In order to avoid centrifugal force effects on the pressurized oil, the engagement actuator can be advantageously configured to be stationary.
Control measures for an individual friction clutch, namely disengagement and engagement procedures, are other advantageous features of the present invention. Gear changing with control of the two friction clutches in a twin clutch gearbox can take place such that the friction clutches are closed by spring force during steady-state operation. This spring force corresponds to the maximum transmittable clutch torque.
When the gear-changing procedure is initiated, the disengagement actuator of the free friction clutch of the following gear is first subjected to pressure so that the next gear can be synchronized. The engagement actuator on the power-transmitting friction clutch is then subjected to a pressure which corresponds to the instantaneous clutch load. The disengagement actuator is then subjected to maximum pressure so that the engagement spring is compressed.
It is not necessary to control the pressure in the disengagement actuator. It must always be higher than that needed to compress the engagement spring. After load is removed from the engagement spring, the engagement actuator takes over the torque transmission. The rest of the gear-changing procedure corresponds to that of conventional automatic gearboxes. After the end of the gear-changing procedure, the operating pressure on both disengagement actuators is switched off and the engagement spring takes over the torque transmission. The engagement actuator pressure can therefore also be switched off so that the entire gear-changing element is unpressurized in steady-state operation.
The friction clutch according to the present invention can be either a dry or a wet clutch. The disengagement actuator can, for example, be fed from a pressure reservoir. Because the friction clutches are closed during steady-state operation, the clutch control can be structured such that both actuators always remain full. This makes the conventional usual filling phase unnecessary and shortens the gear-changing time. The control is advantageously structured such that the engagement actuator is switched on simultaneously with the synchronization of the next gear. This reduces the gear-changing time. The pressure piece of the disengagement actuator can contain the operating piston of the engagement actuator. A corresponding method of the present invention is matched to the above-mentioned control measures for a twin-path gearwheel change-speed gearbox with two friction clutches.